In recent years, efforts to efficiently utilize energy are being made by using a secondary battery such as a lithium-ion battery for a power supply system of a vehicle or a power supply system of a smart house. Increasing the charging/discharging amount of the secondary battery is often effective for enhancing the energy efficiency of a system as a whole. However, it is known that increasing the charge/discharge amount causes the deterioration in characteristics of the secondary battery. In particular, there arises a problem that the internal resistance of the secondary battery increases, and consequently input-output characteristics deteriorate.
Thus, there is an inverse relationship between the instantaneous energy efficiency of the system as a whole and the deterioration in characteristics of the secondary battery. Meanwhile, a use period of the power supply for the above use extends over a long period. Therefore, it is preferable to use the secondary battery in such a manner that the energy efficiency over the whole assumed use period becomes the maximum. For this purpose, it is necessary to control the charge/discharge amount of the secondary battery in such a manner that the deterioration in characteristics of the secondary battery falls within a predetermined range.
It is known that, in general, the deterioration in characteristics of the secondary battery progresses fast when a voltage of the secondary battery is too high or too low. In actuality, the voltage of the secondary battery is a difference between a positive electrode potential and a negative electrode potential of the secondary battery, and therefore the speed of the deterioration in characteristics is determined not by a battery voltage but by the positive electrode potential and the negative electrode potential. Therefore, correctly detecting the positive electrode potential and the negative electrode potential of the secondary battery, and then selecting the optimum battery operation according to the detected electric potentials, is effective as a means for suppressing the deterioration. For example, PTL 1 discloses a method in which deterioration states of a positive electrode, a negative electrode and an electrolyte solution are quantitatively evaluated in a nondestructive manner by using a charging/discharging curve of the secondary battery to determine an open circuit potential of the positive electrode and an open circuit potential of the negative electrode.
PTL 1 describes a state determination method for determining a state of a secondary battery, and describes a method in which a charging/discharging curve of the secondary battery is reproduced by calculation on the basis of separate charging/discharging curves of a positive electrode and a negative electrode stored beforehand, and in the process of the reproduction, an effective weight of a positive electrode active material, an effective weight of a negative electrode active material, a deviation in capacitance between the positive electrode and the negative electrode, and a positive electrode potential and a negative electrode potential corresponding to an open circuit voltage of the secondary battery are obtained. In addition, PTL 1 indicates that the secondary battery is controlled by using the positive electrode potential and the negative electrode potential that have been obtained, which enables to achieve a higher degree of safety of the secondary battery, and to suppress the deterioration in characteristics in comparison with the control based on the battery voltage in the prior art.